Material level detector circuit

Abstract

Disclosed are apparatus and methods for detecting the presence, or absence, of material at a predetermined location. An oscillator generates a constant-frequency, constant-amplitude signal which is fed through a load resistor to an antenna probe positioned at the sensing location. The amplitude of the signal fed to the antenna probe varies as the impedance of the antenna probe is affected by material in its environment, or lack thereof. The signal amplitude is converted into a d-c voltage level, and is transmitted to a switching circuit which provides an output signal depending on a comparison between the d-c voltage level and a reference voltage level. The sensitivity of the switching circuit is adjustable to accommodate the degree to which the particular material, whose presence or absence is to be detected, affects the impedance of the antenna probe. The output from the switching circuit may be used to operate one or more devices indicative of the environment of the antenna probe and/or affecting the environment by adding to and/or removing therefrom material affecting the impedance of the antenna probe.

I claim: 
     
       1. Apparatus for detecting the presence or absence of material at a location comprising: (a) signal generation means for providing an oscillatory signal;   (b) antenna probe means for positioning at said location, and for receiving said oscillatory signal;   (c) converter means for producing a voltage level whose value is dependent on the amplitude of said oscillatory signal;   (d) switching means, operable by said voltage level, for producing an output signal dependent on the presence or absence of material at said location;   (e) comparator means, as part of said switching means, whose output controls said output signal depending on the value of said voltage level produced by said converter means compared to a reference voltage level;   (f) sensitivity control means, for selectively adjusting the sensitivity of response of said switching means, including voltage selection means for selectively providing voltage levels to said switching means to selectively determine the value of said reference voltage level whereby the magnitude of the variation of said voltage levl, produced by said converter means, required to effect a change in said output signal is so selectively determined; and   (g) said voltage selection means further comprising a voltage divider including a plurality of resistors, and sensitivity switch means for selectively communicating voltages from contacts between resistors in said voltage divider to said switching means to so selectively determine the value of said reference voltage level.   
     
     
       2. Apparatus as defined in claim 1 further comprising an impedance bridge, including said antenna probe means, for providing input to said converter means that is dependent in amplitude on the impedance load of said antenna probe means. 
     
     
       3. Apparatus as defined in claim 2 wherein said oscillatory signal is so provided by said signal generation means as a constant-frequency, constant-amplitude signal. 
     
     
       4. Apparatus as defined in claim 3 wherein said switching means further comprises feedback means whereby said voltage level is increased or decreased, depending on the value of said comparator output, to define a voltage value range through which said voltage level must vary to cause said switching means output signal to vary. 
     
     
       5. Apparatus as defined in claim 4 wherein said switching means further comprises calibration means, and said impedance bridge further comprises variable resistance means, such that said voltage level may be adjusted relative to said reference voltage by variation of said variable resistance means, and whereby said calibration means indicates the condition of such adjustment. 
     
     
       6. Apparatus as defined in claim 5 wherein said signal generation means comprises: (a) first inverter circuit means for generating said oscillatory signal;   (b) means for stabilizing the frequency of said oscillatory signal; and   (c) second inverter circuit means for increasing the load capability of said oscillatory signal.   
     
     
       7. Apparatus as defined in claim 6 further comprising control unit means for receiving said output signal so produced by said switching means, and for controlling ancillary apparatus in response to said output signal. 
     
     
       8. Apparatus as defined in claim 1 wherein said switching means further comprises calibration means, and said impedance bridge further comprises variable resistance means, such that said voltage level may be adjusted relative to said reference voltage by variation of said variable resistance means, and whereby said calibration means indicates the condition of such adjustment. 
     
     
       9. Apparatus as defined in claim 1 wherein said switching means further comprises feedback means whereby said voltage level is increased or decreased, depending on the value of said comparator output, to define a voltage value range through which said voltage level must vary to cause said switching means output signal to vary. 
     
     
       10. Apparatus as defined in claim 1 wherein said switching means further comprises feedback means whereby said voltage level is increased or decreased, depending on the value of said comparator output, to define a voltage value range through which said voltage level must vary to cause said switching means output signal to vary. 
     
     
       11. Apparatus as defined in claim 1 further comprising control unit means for receiving said output signal so produced by said switching means, and for controlling ancillary apparatus in response to said output signal. 
     
     
       12. Apparatus as defined in claim 1 wherein said output signal is digital. 
     
     
       13. Apparatus as defined in claim 1 wherein said output signal is analog. 
     
     
       14. A system for monitoring the level of material in a container comprising: (a) antenna probe means for location within said container;   (b) signal generation means for generating a constant amplitude oscillatory signal for loading said antenna probe means;   (c) converter means for receiving said oscillatory signal so loaded on said antenna probe means, and for producing a voltage level dependent on the amplitude of said oscillatory signal so received;   (d) switching means for receiving said voltage level so produced by said converter means, and providing an output signal dependent on the value of said voltage level compared to a reference level;   (e) sensitivity control means for selectively adjusting the value of said reference level to so selectively adjust the sensitivity of response of said switching means, whereby the magnitude of the variation of said voltage level, produced by said converter means, required to effect a change in said output signal is so selectively determined; and   (f) said sensitivity control means further comprising a voltage divider including a plurality of resistors, and sensitivity switch means for selectively communicating voltage from contacts between resistors in said voltage divider to said switching means to so selectively determine the value of said reference level.   
     
     
       15. A system as defined in claim 14 wherein: (a) said switching means comprises comparator means for comparing said voltage level, produced by said converter means, to said reference level;   (b) said switching means further comprises feedback means whereby said voltage level is increased or decreased, depending on said comparator output, to thereby define a voltage range through which said voltage value so produced by said converter means must vary to cause said comparator output to vary;   (c) said comparator output takes on values depending on the comparison of said voltage level with said reference level; and   (d) said output signal so produced by said switching means is dependent on the value of said comparator output.   
     
     
       16. A system as defined in claim 15 further comprising: (a) variable resistor means cooperating with said antenna probe means to form an impedance bridge on which said oscillatory signal is impressed, whereby said converter means receives said oscillatory signal form a point between said variable resistor means and said antenna probe means; and   () calibration means, as part of said switching means, whereby said variable resistor means may be selectively adjusted to determine and control the value of said voltage level relative to said reference level, and whereby said calibration means indicates the relationship between said voltage level and said reference level.   
     
     
       17. A system as defined in claim 16 further comprising control unit means for receiving said switching means output signal, and for controlling ancillary apparatus in response thereto. 
     
     
       18. A system as defined in claim 17 wherein said ancillary apparatus includes means for adding material to said container. 
     
     
       19. A system as defined in claim 17 wherein said ancillary apparatus includes means for removing material from said container. 
     
     
       20. A system as defined in claim 17 wherein said ancillary apparatus includes means for indicating the presence or absence of material at said antenna probe means. 
     
     
       21. A system as defined in claim 16 wherein said output signal is digital. 
     
     
       22. A system as defined in claim 16 wherein said output signal is analog. 
     
     
       23. A system as defined in claim 14 wherein said oscillatory signal is so generated by said signal generation means as a constant-frequency, constant-amplitude signal. 
     
     
       24. A system as defined in claim 14 wherein said signal generation means comprises: (a) first inverter circuit means for generating said oscillatory signal;   (b) means for stabilizing the frequency of said oscillatory signal; and   (c) second inverter circuit means for increasing the load capability of said oscillatory signal.   
     
     
       25. A system as defined in claim 14 wherein: (a) said switching means comprises comparator means for comparing said voltage level, produced by said converter means, to said reference level;   (b) said switching means further comprises feedback means whereby said voltage level is increased or decreased, depending on said comparator output, to thereby define a voltage range through which said voltage value so produced by said converter means must vary to cause said comparator output to vary;   (c) said comparator output takes on values depending on the comparison of said voltage level with said reference level; and   (d) said output signal so produced by said switching means is dependent on the value of said comparator output.   
     
     
       26. A system as defined in claim 14 further comprising: (a) variable resistor means cooperating with said antenna probe means to form an impedance bridge on which said oscillatory signal is impressed, whereby said converter means receives said oscillatory signal from a point between said variable resistor means and said antenna probe means; and   (b) calibration means, as part of said switching means, whereby said variable resistor means may be selectively adjusted to determine and control the value of said voltage level relative to said reference level, and whereby said calibration means indicates the relationship between said voltage level and said reference level.   
     
     
       27. A system as defined in claim 14 further comprising control unit means for receiving said switching means output signal, and for controlling ancillary apparatus in response thereto. 
     
     
       28. A system as defined in claim 27 wherein said ancillary apparatus includes means for adding material to said container. 
     
     
       29. A system as defined in claim 27 wherein said ancillary apparatus includes means for removing material from said container. 
     
     
       30. A system as defined in claim 27 wherein said ancillary apparatus includes means for indicating the presence or absence of material at said antenna probe means. 
     
     
       31. A system as defined in claim 14 wherein said output signal is digital. 
     
     
       32. A system as defined in claim 14 wherein said output signal is analog. 
     
     
       33. A system as defined in claim 14 further comprising at least one additional combination of antenna probe means, signal generation means, converter means, switching means, and sensitivity control means, as defined in claim 20, wherein said antenna probe means is locatable at a level different from that of the first antenna probe means of claim 20. 
     
     
       34. A system as defined in claim 33 further comprising control means for receiving the output signals from the switching means of said systems, and for controlling ancillary apparatus in response thereto. 
     
     
       35. A system as defined in claim 34 wherein said ancillary apparatus includes means for adding material to said container. 
     
     
       36. A system as defined in claim 34 wherein said ancillary apparatus includes means for removing material from said container. 
     
     
       37. A system as defined in claim 34 wherein said ancillary apparatus includes means for indicating the presence or absence of material at said antenna probe means. 
     
     
       38. A method of monitoring the presence or absence of material at a location comprising the following steps: (a) providing an antenna probe positioned at said location;   (b) providing a variable resistor, cooperating with said antenna probe to form an impedance bridge;   (c) providing a converter circuit for converting oscillatory signals to voltage level values dependent on the amplitude of the oscillatory signal so converted;   (d) providing a switching circuit for receiving such a voltage level value, and comparing said voltage level value with a reference level, and producing an output signal dependent on said comparison;   (e) providing feedback means, as part of said switching circuit, for defining a voltage range through which said voltage level must vary to alter said output signal;   (f) providing sensitivity control means for selectively adjusting the value of said reference level;   (g) impressing an oscillatory signal on said impedance bridge; transmitting said oscillatory signal from a point on said impedance bridge, between said variable resistor and said antenna probe to said converter circuit for conversion thereby to a voltage level; and applying said voltage level to said switching circuit; and   (h) generating an output signal by said switching circuit in accordance with the comparison of said voltage level with said reference level.   
     
     
       39. A method as defined in claim 38 further comprising the additional step of adjusting said variable resistor, with no material to be detected at said location, so that said voltage level is just above the value of said reference level. 
     
     
       40. A method as described in claim 38 further comprising the additional step of adjusting said sensitivity control means to alter the value of said reference level. 
     
     
       41. A method as described in claim 38 further comprising the additional steps of: (a) providing control means for receiving said output signal; and   (b) controlling ancillary apparatus by said control means in response to said output signal.   
     
     
       42. A method as described in claim 41 comprising the additional step of employing said ancillary apparatus for adding material generally at said location. 
     
     
       43. A method as described in claim 41 comprising the additional step of employing said ancillary apparatus for removing material generally from said location. 
     
     
       44. A method as described in claim 38 further comprising the additional steps of carrying out the steps of claim 38 for at least one additional antenna probe at a different location to produce at least one additional output signal. 
     
     
       45. A method as described in claim 44 further comprising the additional steps of: (a) providing control means for receiving said output signals; and   (b) controlling ancillary apparatus by said control means in response to said output signals.   
     
     
       46. A method as described in claim 45 comprising the additional step of employing said ancillary apparatus for adding material generally at said locations. 
     
     
       47. A method as described in claim 45 comprising the additional step of employing said ancillary apparatus for removing material generally from said locations.